The invention relates to a moisture sensor for large-area layers utilizing conductors disposed in the layers.
Waste deposits generally have clay-mineral bottom layers providing a seal to prevent leaching of the waste by rain and the passage of leach water into the ground water. These sealing layers, that is, the base seal layer at the bottom as well as the cover layer on top are resilient so that they can follow any deformation of the whole waste pile and remain sealed for many decades. However, the sealing layer may rupture by uncontrollable shearing stresses. Also, a relatively small loss of moisture (about 4-5%) from this highly densified and almost saturated mineral protective layer results in a loss of its ability to provide water insulation. An early recognition and localization of a possible fracture or drying out of a damaged area is therefore of great environmental and economical importance. The usual perk tests made today indicate a damage too late and do not permit to determine the location of the damaged area. A reproducible location indicating measurement of the moisture and the density of the sealing layer is required, but is not available up to this date.
In farm and nursery operations, an optimal irrigation depends on the moisture measurement of a large ground area with a high local resolution. Too little irrigation causes damage to the plants, too much irrigation results in washing away of nutrients and in salt deposits.
In large grain storage areas, it is important to monitor the moisture of the whole volume of stored grain. Samples taken from the surface or measurements taken on the surface provide insufficient measurement accuracies.
Meteorological models require an accurate knowledge of the water content of the soils since the energy exchange between the ground and the atmosphere occurs mainly by evaporation and condensation of water. This involves particularly the layers close to the surface.
For moisture measurements, generally probes are used which determine the dielectric coefficient DK of the mixture to be tested. From this, the moisture can be determined with the aid of calibration measurements. A particular procedure which is in use is a time-based signal reflection method, which, in the technical literature, is generally called "Time-Domain-Reflectometry" method or abbreviated "TDR" method.
This method is based on the travel of electromagnetic waves in conductors. The electric properties of conductors and, consequently, the signal transmission characteristics are defined, among others, by the characteristic impedance the attenuation, and the travel speed of the signal. As is well known, these values depend (possibly with losses) on the dielectric properties of the ambient space in which the electric field extends. This space is limited in co-axial conductors by the outer shielding. In open conductor systems such as the double conductors (Lecher-conductor), the triple or multiple conductor and the surface wave guide (for example, according to Gouban), however the field extends beyond the close surroundings of the conductor. A change in the material properties in that area also changes the signal transmission properties of the conductor.
The Time Domain Reflectometry (TDR) method for measuring the moisture content in soils is known from the publication "Wasser & Boden" (R. Rook, S. Melchior, G. Miehlich, "DIE TIME DOMAIN REFLECTOMETRY (TDR) FUR WASSERGEHALTSMESSUNG IN BODEN", Wasser & Boden, Nov. 4, 1993). The probe disclosed therein is based, among others, on the sensor introduced by Davis et al., (J. L. Davis, A. P. Amman, "ELECTROMAGNETIC DETECTIONS OF SOIL MOISTURE", Canadian Journal of Remote Sensing, Vol. 3, No. 1, 1977a).
A disadvantage of this sensor is the short measurement length of the measurement fork forming an open conductor. Generally, TDR probes are made and offered in lengths of only a few 10 centimeters. It is also disadvantageous that different layers along the probe rod can average out the results. Multiple reflections can neutralize one another. Also, the relative high electric conductivity limits the application to a few centimeters because of a high conductor attenuation. Another disadvantage is that the fork may spread apart when it is inserted into a mixture. The impedance increase resulting therefrom indicates a drier state than is actually present. The dielectricity coefficient of water is degraded greatly at frequencies greater than 1 GHz. This degradation is subject to many additional factors. In order to obtain an accurate correlation between the DK and the moisture content the measuring frequency (and consequently, the ramp steepness of a measuring pulse) must be limited to less than 1 GHz. The TDR probes known so far unfortunately have no such limitation.
It is the object of the present invention to provide a sensor of the type described above which however covers a relatively large area and has a good location resolution.